This invention relates to fasteners. More particularly but not exclusively, the invention relates to fasteners for fastening wall elements to wall structures of gas turbine engine combustors.
The designers of gas turbine engine combustors are required to meet stringent emission targets. This means that the temperatures within the combustors will need to increase, as cooling air is diverted to the inside of the combustor to control emissions.
The present combustors are formed with a tiled inner wall, but such tiles have a maximum temperature threshold of about 1150xc2x0 C.
According to one aspect of this invention, there is provided a fastener for fastening a first article to a second article at least the first article being in an elevated temperature environment, the fastener comprising an elongate main part which can extend through the first and second articles, wherein the main part defines a cooling pathway therethrough to allow the passage of a cooling gas through the main part.
Preferably, the cooling aperture extends lengthways through the said main part.
In the preferred embodiment, the main part comprises a shank and a head. The pathway may extend through the shank and through the head.
According to another aspect of this invention, there is provided a fastener comprising a main part having a head and a shank wherein a pathway is defined through the head and shank to allow the passage of a gas through the main part.
The pathway may be in the form of an aperture extending through the main part.
The shank is preferably formed to cooperate with a further part to secure the second article to the main part. The shank may be threaded and the further part may be a nut.
Preferably, the main part comprises a bolt.
The preferred embodiment is particularly suitable for use in fastening a first article which may be in the form of a main body of a wall element to a second article which may be in the form of wall of a gas turbine engine combustor. The main body may be in the form of a wall tile. The main body may be formed of a ceramic material.
Preferably the head can engage the first article which may be generally planar in configuration. The first article may be the main body of a wall element.
The head is preferably shaped to co-operate with the first article to allow differential thermal expansion between the first article and the main part of the fastener.
The head may be of a generally conical configuration and may taper outwardly from a shank. The taper of the head may vary depending upon the difference in thermal expansion coefficients in the in plane and perpendicular directions in the first article. Where the first article has isotropic thermal expansion coefficients, the taper may be substantially 45xc2x0 to the longitudinal axis of the shank. Where the first article has at least one anisotropic thermal expansion coefficients, the taper may curve inwards or outwards. The head preferably has an inner surface which, in use, faces inwardly of the combustor. The inner surface is preferably coated with a thermal barrier coating.
In another embodiment the section of the cooling pathway extending through the head is preferably angled relative to the section extending through the shank. In another embodiment, the cooling aperture extends generally parallel to, and preferably substantially coaxially with, the main longitudinal axis of the main part. The fastener preferably includes a spacer to space the first article from the second article. The main part conveniently extends through the spacer.
The spacer may be in the form of a further nut threadably receivable on the shank. The fastener may further include load spreading means arrangeable on the shank between said spacer and the first article. The load spreading means may comprise a washer.
According to another aspect of this invention there is provided a wall element for a wall structure of a gas turbine engine, the wall element comprising a main body and a fastener as described in paragraphs four to fifteen above to fasten the main body to an outer wall of the combustor.
The material from which the main body is formed may be a ceramic material, for example a ceramic matrix composite. An example of a suitable ceramic matrix composite is silicon carbide fibre/silicon carbide matrix composite other examples include alumina fibre/alumina matrix composites and mullite fibre/mullite matrix composites or mixtures of the foregoing. Such a material may have a temperature limit of greater than 1200xc2x0. The main body preferably has an inner surface which, in use, faces inwardly of the combustor. The inner surface of the main body is preferably coated with a thermal barrier coating.
The main body of the wall element may define at least one aperture to receive the fastener therethrough. The aperture may have a conical side which may taper inwardly from an inner surface of the main body to an outer surface thereof. Preferably, the shape of the side of the aperture corresponds to the shape of the head of the securing member. Advantageously, the head of the securing member, and the side wall of the aperture taper at substantially the same angle. This tapering of the head and the aperture provides the advantage in the preferred embodiment that the differential thermal expansion between the main body and the securing member is accommodated by the sliding of the head against the wall of the recess.
According to another aspect of this invention there is provided a wall structure for a combustor of a gas turbine engine, the wall structure comprising inner and outer walls, the inner wall comprising a plurality of wall elements as described in paragraphs sixteen to eighteen above.
According to another aspect of this invention there is provided a combustor for a gas turbine engine including radially inner and outer wall structures as described in paragraph nineteen above.